GitLab

Toggle "closed" class when open/close menu to achieve transition animation.
This CL only has opening animation.

TODO: Add close animation; Add wave like animation to label when opening menu.

Bug: 814507

Change-Id: I2a036297741f34905fe868c7ea0a99e9ea91cea3
Reviewed-on: https://chromium-review.googlesource.com/c/1286949
Commit-Queue: Jazz Xu <jazzhsu@google.com>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603116}